Titanium is a lightweight material whose specific gravity is as low as about half that of steel and which is characterized by its high corrosion resistance and high strength. Titanium is therefore used for parts of aircrafts, railway vehicles, two-wheeled vehicles, automobiles, etc. for which reduction in weight is greatly desired, home appliances, members for construction, etc. Titanium is also used as a material for medical use because of its high corrosion resistance.
However, applications of titanium are limited due to its high material cost, as compared to iron and steel materials and aluminum alloys. In particular, titanium alloys have tensile strength as high as more than 1,000 MPa, but do not have enough ductility (elongation to failure). Moreover, titanium alloys have poor plastic workability at normal temperature or in a low temperature range. Pure titanium has elongation to failure as high as more than 25% at normal temperature and has excellent plastic workability in a low temperature range. However, pure titanium has tensile strength as low as about 400 to 600 MPa.
Various studies have been carried out in response to a very strong need for titanium having both high strength and high ductility and for reduction in material cost of titanium. In particular, many techniques of strengthening titanium by using relatively inexpensive elements such as oxygen and nitrogen rather than expensive elements such as vanadium, scandium, and niobium have been studied as related art in order to achieve cost reduction.
For example, Journal of the Japan Institute of Metals and Materials, Vol. 72, No. 12 (2008), pp. 949-954 (Non-Patent Literature 1), entitled “Effect of Nitrogen on Tensile Deformation Behavior and Development of Deformation Structure in Titanium,” describes the use of nitrogen as an alloy element for titanium alloys. Specifically, Non-Patent Literature 1 describes that titanium sponge and TiN powder are weighed to predetermined compositions and are arc-melted to produce Ti—N alloys with various nitrogen concentrations. In this case, both high strength and high ductility can be achieved if a homogenous solid solution of nitrogen atoms in a Ti matrix is formed.
Another method is a technique of adding TiN particles to molten Ti to form a solid solution of nitrogen atoms in a Ti matrix when the mixture of TiN particles and molten Ti solidifies. In this case as well, both high strength and high ductility can be achieved if a homogenous solid solution of nitrogen atoms in the Ti matrix is formed.